As methods for producing DL-homocystine, methods using inexpensive and easily available DL-methionine as a raw material are known. For example, J. Biol. Chem., 99, 135 (1932-33) describes a method of producing DL-homocystine by heating DL-methionine in a large excess of sulfuric acid. Further, a method of producing DL-homocystine comprising reducing DL-methionine with metallic sodium in liquid ammonia to DL-homocysteine, followed by oxidization thereof is also known (West German Patents 3,309,761 and 2,547,672).
The present inventors have proposed a method of reacting methionine with sulfuric acid in the coexistence of a hydrogen halide (JP-A-10-204055 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"): details thereof will be described below).
However, homocystine produced by these methods contains by-products such as polysulfides in small amounts, particularly, L-homocystine further contains by-products such as mesohomocystine in small amounts in addition. Accordingly, it cannot be used as it is as an intermediate for synthesizing various organic compounds including medicines and agricultural chemicals. For obtaining a high purity product, a further purifying process of crystallization is required. However, repetition of simple crystallization cannot provide the high purity product, and up to the present time, an industrial purifying method for obtaining the high purity product has not been established.